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Full text of "Handbook Of Chemical Engineering - I"

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280
CHEMICAL ENGINEERING
less than 4,375 sq. ft. of ground space while a single tank of tqual settling area requires about 2,500 sq. ft. of floor space.
The goose neck discharge makes for reduction in head room but tends to choke, this tendency increasing as the material discharged becomes coarser. Most users of the tank prefer to run it without the goose neck extension and to throttle down by valves or plugs at the bottom of the tank with openings of suitable size. The loss
of head room causes no reduction in density of pulp. Where the goose neck extension is used a proportionately larger discharge opening must be used than would be indicated by the less head merely owing to the necessity of providing for extra friction in the goose-neck extension and reasonable freedom from chokages likely to occur from passing the discharging material through the comparatively great length of small pipe. For many years there existed much confusion of ideas among users of thickening of discharge. It was thought that a small /ter, increased the thickening effect and also ocd as the head became greater it was thought
FIG. 11.óDorr thickener.
that the head of discharge should be as small as possible. In any settlement apparatus there are zones of settlement, the rate in the zones near the surface of the liquid being fastest and that of the bottommost being slowest. In the lowest zone there is the greatest density of pulp and the slowest settlement and compacting of the settling material. It is evident then that if R is the rate of formation of this lowermost zone and A the area of the tank in which the material is being settled then RA is the cubical content rate at which the settling material can be discharged in its greatest